Heritage speakers exhibit a tendency to overgeneralize morphological paradigms in their production, but little is known about their perception, and the present study seeks to better understand how heritage speakers process new information in their home language. To this end, 119 listeners from different language backgrounds evaluated the number of syllables in 70 nonce words, all four-syllable paroxytone nonce words with an initial obstruent + vowel + flap sequence, with the first vowel presented at 100%, 75%, 50%, and 25% of its original duration. Two types of word endings were used: -fono, designed to be reminiscent of the word teléfono, a high frequency four-syllable word, and -pine, which does not clearly prime any existing Spanish words, e.g. teréfono and terépine, respectively. Cumulative link mixed effects models fitted to 119 participants’ evaluations (N = 32,619) show that heritage speakers evaluate nonce words ending in -fono as four syllables significantly more than words ending in -pine (p < 0.001), but no word-ending effect was found for any other language group. I contend that heritage speakers filter new information in their heritage language through existing knowledge more rigidly than other Spanish speakers, and these overgeneralizations in perception parallel heritage speakers’ overgeneralizations in production.